United Kingdom

Solid-state quantum emitters have great promise for quantum information applications such as, quantum computation and communication. Nitrogen-Vacancy (NV) centers in diamond are popular candidate due to its long-lived spin quantum state and well-defined optical transitions. Limitations of NV centers arise from the strongly temperature dependent dephasing effects caused by phonon interactions, which act to suppress the indistinguishability of photon emission essential for quantum interference. 
In this work we simulate the design of a cavity with an ultrasmall-mode-volume coupled to NV-center and show that we can extract highly efficient (99%) indistinguishable (&gt;50%) photons at 200K. Upon filtering, we find an enhanced indistinguishability (73%) with efficiency (29%). We use a cavity-QED master equation approach to calculate efficiencies and indistinguishability of the photon emission at 200K and 300K. Typically, NV-centers are modelled as a two-level system with excess pure-dephasing. However, the NV-center consists of two orthogonally polarised dipoles which are non-degenerate in the presence of linear strain fields. To capture this polarisation splitting we model the NV-center as both a two- and three-level system and find that the inclusion of the non-degenerate excited states supresses indistinguishability and the results are optimized when the cavity orientation is aligned to one of these transition dipoles.


Bristol Quantum Information Technologies Workshop

Efficient indistinguishable photons at 200K

nv-center

single photon sources

cavity qed

**BQIT is a three day annual workshop** aimed at enabling leading UK and international academics and industrial partners to come together; to explore and discuss future ambitions and challenges in the field of **Quantum Information Technologies.** Since our conception in 2014 we have had 230 speakers and panellists who have presented their work and opinions on a range of topics, from quantum theory to innovation in industry.


### Welcome Message
The BQIT:21 team welcomes you to the eighth Bristol Quantum Information Technologies workshop! 

Last year was the first time in our workshop's history that we had gone fully virtual. The global circumstances that continue to impact us all professionally and personally have forced BQIT once again to be delivered as an all-virtual workshop for 2021. The access this creates to colleagues across the world who would otherwise be unable to join us, as well as being able to make the workshop free for all attendees due to the lower costs involved in delivering an online workshop, are just a couple of the extraordinary benefits of meeting online. This also means that we can welcome three times as many attendees to BQIT than previous years.   

We take this opportunity to thank each of our speakers and poster presenters for contributing their technical content, this year's sponsors, whose support this year enabled BQIT:21 to be free to all attendees, and to all of our attendees for joining us online this year. We recognise the extra energy required to stay connected and motivated during these difficult times, and are hugely encouraged to see so many of you willing to continue to connect with us on an online platform. 

It is our hope that you have a productive and enjoyable time while you engage and interact with BQIT:21, and to everyone that has helped take BQIT:21 online, thank you! 

**The BQIT board and organisation team **


![](https://assets.underline.io/uploads/markdown_image/1/image/a12e0c1ad3410144cb48557690e4ecaf.png)
### This year's workshop agenda is available here: [Virtual BQIT:21](https://drive.google.com/file/d/1QwsSM0ztnxC_lgUKf3TbO3GvnI44AQkY/view?usp=sharing)
For further information, please contact us at: **bqit-admin@bristol.ac.uk**

![](https://assets.underline.io/uploads/markdown_image/1/image/a12e0c1ad3410144cb48557690e4ecaf.png)

Solid-state quantum emitters have great promise for quantum information applications such as, quantum computation and communication. Nitrogen-Vacancy (NV) centers in diamond are popular candidate due to its long-lived spin quantum state and well-defined optical transitions. Limitations of NV centers arise from the strongly temperature dependent dephasing effects caused by phonon interactions, which act to suppress the indistinguishability of photon emission essential for quantum interference. 
In this work we simulate the design of a cavity with an ultrasmall-mode-volume coupled to NV-center and show that we can extract highly efficient (99%) indistinguishable (>50%) photons at 200K. Upon filtering, we find an enhanced indistinguishability (73%) with efficiency (29%). We use a cavity-QED master equation approach to calculate efficiencies and indistinguishability of the photon emission at 200K and 300K. Typically, NV-centers are modelled as a two-level system with excess pure-dephasing. However, the NV-center consists of two orthogonally polarised dipoles which are non-degenerate in the presence of linear strain fields. To capture this polarisation splitting we model the NV-center as both a two- and three-level system and find that the inclusion of the non-degenerate excited states supresses indistinguishability and the results are optimized when the cavity orientation is aligned to one of these transition dipoles.


poster

Global quantum communications enable networked quantum information processing. Satellite QKD (SatQKD) overcome current optical fibre range limitations, but limited transmission times between satellites and ground stations constrains the amount of secret key due to finite-block size effects. We analyse these effects and their implications for system design and operation, for a trusted-node downlink employing an optimised BB84 with weak coherent pulse decoy states.

Finite Key Effects in Satellite-based Quantum Key Distribution

Introduction to the Angstrom Engineering Quantum Series Platform

Quantum Series Platform - Angstrom Engineering

Sponsor demo - IDQuantique

I present an overview of the LIGENTEC platform that provides new paths for quantum applications

LIGENTEC - Photonic Integrated Ciruits for quantum applications

Sponsor demo - University of Bristol

In this talk, I will present the main product of QuiX: a quantum photonic processor, i.e. a large-scale tunable multimode optical interferometer, for use in photonic quantum information processing applications. QuiX has recently demonstrated the world's largest photonic processor, and I will present results on the characterization, calibration and operation of this device.

QuiX Quantum Photonic Processors

EPJ Quantum Technology is part of the well known European Physical Journal (EPJ) series, indexed in all major citation databases, which covers the entire spectrum of pure and applied physics. We will give a brief introduction to the journal’s aims and scope, editorial structure, articles types and the current special issues, followed by an overview of recently published highlights.

An introduction to EPJ Quantum Technology journal.

Zurich Instruments continues to develop instrumentation for quantum technologies with growing demand for high measurement fidelity, control speed, and reliability as well as scalability. In this presentation, I will introduce the latest arbitrary waveform generation (AWG) product SHFSG and quantum analyzer SHFQA that can be seamlessly integrated into a system solution QCCS (Quantum Computing Control System) capable of working with both large and small quantum systems.

Next generation instruments for quantum control and readout from Zurich Instruments

In this talk I will briefly review and discuss a recent no-go theorem on an extended version of the Wigner’s friend paradox [1]. In that work, we showed that a set of metaphysical assumptions we called “Local Friendliness” lead to inequalities that can be in principle violated by quantum correlations. We demonstrated this violation in a “proof of principle” experiment. I will discuss how violation of Local Friendliness puts strictly stronger constraints on physical theories than Bell's theorem. I will then discuss what an ultimate experiment may look like, some implications of this result for quantum foundations, and future research directions.
[1] Bong et al, Nature Physics 16, 1199 (2020)


What’s ahead for Wigner’s friend?

Device-independent quantum key distribution is a secure quantum cryptographic paradigm that allows two honest users to establish a secret key, while putting minimal trust in their devices. Most of the existing protocols have the following structure: first, a bipartite nonlocal quantum state is distributed between the honest users, who perform local projective measurements to establish nonlocal correlations. Then, they announce the implemented measurements and extract a secure key by post-processing their measurement outcomes. We show that no protocol of this form allows for establishing a secret key when implemented on certain entangled nonlocal states, namely on a range of entangled two-qubit Werner states. To prove this result, we introduce a technique for upper-bounding the asymptotic key rate of device-independent quantum key distribution protocols, based on a simple eavesdropping attack. Our results imply that either different tools are needed for device-independent quantum key distribution in the large-noise regime, or Bell nonlocality is not sufficient for this task.

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